Catalyst for removing nitrogen oxides, method for production thereof and method for removing nitrogen oxides

ABSTRACT

Disclosed is a method for producing a catalyst for removing nitrogen oxides which comprises dispersing a hydrated titanium (Ti) oxide or dried material thereof, tungstic acid or a salt thereof, and cerium (Ce) dioxide in a dispersion medium to form a sol-like material, mixing the sol-like material with an aqueous medium to form a catalyst slurry or paste, supporting the catalyst slurry or paste on a catalyst carrier, and then calcinating the carrier; in which catalyst the Ce dioxide is prevented from being embedded in the Ti oxide to realize such a high degree of dispersion of the Ce dioxide on the surface of the Ti oxide as comparable with the case wherein cerium ions are dispersed in micro voids of a zeolite by ion exchange; and the catalyst is free from the occurrence of such phenomena as sintering of the Ti oxide, and deterioration of zeolite with steam when a zeolite is used as carrier.

TECHNICAL FIELD

The present invention relates to a catalyst used for removing nitrogenoxides (NOx) contained in an exhaust gas, a method for producing thecatalyst, and a method for removing NOx. More specifically, the presentinvention relates to a catalyst used for removing NOx contained in anexhaust gas having a temperature of higher than 450° C., a method forproducing the catalyst, and a method for removing NOx contained in anexhaust gas.

BACKGROUND ART

In the United Sates of America and the likes in which a large amount ofelectricity is consumed, the generation of electric power using theso-called simple cycle gas turbines, for which gas turbines areinstalled and operated independently, has been increased in order tosupplement electricity and to cope with a time when the amount ofelectricity used becomes a peak. Since the equipments used for theelectric power generation are constructed in suburbs of towns andcities, it is necessary to decompose NOx contained in exhaust gases at ahigh ratio to purify the gases. On the other hand, in the electric powergeneration using simple cycle gas turbines, it is necessary to install adenitrating apparatus just behind the exit of each gas turbine to treatan exhaust gas at a high temperature of 450° C. to 600° C. However,denitration catalysts having high performances and a long life at such ahigh temperature have not been obtained heretofore. Since the loweringof the activity of catalyst due to its deterioration by heat isremarkable especially at a high temperature region, it was necessary totreat an exhaust gas by using a catalyst having a low activity but anincreased thermal resistance at the sacrifice of activity and packingthe catalyst in an amount several times as large as that used fordenitrating an exhaust gas discharged from a boiler and having atemperature of about 350° C. Accordingly, even in an equipmentconstructed to cope with a time when the amount of electricity usedbecomes a peak and having a short annual operation time, it is necessaryto provide a large denitrating apparatus. Thus, it has come to a heavyburden on society even economically.

Accordingly, many inventions of catalysts which are hardly deterioratedat high temperatures have been made, and it has been known particularlythat catalysts having cerium (Ce) as an active component exhibitcomparatively high performances even at high temperatures. For instance,a catalyst which is prepared by coprecipitating a soluble titanium (Ti)compound, tungsten (W) compound, and cerium (Ce) compound so that theparticles of cerium compound are highly dispersed in titania isdisclosed in Laid-open Japanese Patent Publication No. Hei 08-257402.Further, for intending to increase the activity and stability of acatalyst prepared by dispersing Ce ions within micro voids of a zeoliteby ion exchange method is disclosed in Laid-open Japanese PatentPublication No. Hei 08-27408. Whereas these catalysts have an excellentside from the purpose of increasing the stability and activity ofcatalyst by highly dispersing Ce, which is known as an active componentof a catalyst for a long time, in the catalysts, they have problemsremained to improve from the purpose of providing a catalyst having ahigh activity and an increased heat resistance at a high temperatureregion, pursued by the present inventors.

Among the conventional technologies described above, a method forpreparing a catalyst by precipitating soluble compounds of Ti, W, and Ceby coprecipitation have problems as follows:

-   -   i) As a premise, Ti and Ce have the same valence of 4 and the        compounds comprising one of both the elements, respectively, are        ready to form a state wherein the compounds are uniformly        dispersed in each other when they are mixed. Thus, when mixed,        the compounds form a state wherein a large portion of a Ce        compound (oxide) is embedded in a Ti compound (oxide) so that it        is impossible to sufficiently exploit the excellent activity of        the Ce compound. This conclusion can be inferred from the fact        that the separation of both the compounds from each other is        difficult and a Ce oxide remains in an industrial titanium oxide        frequently in an amount of nearly 1% as impurity.    -   ii) A precipitate obtained by coprecipitation method presents a        gel-like state and can hardly be subjected to filtration. Thus,        it is necessary to pass the gel-like precipitate through many        complicated steps by the time when the precipitate can actually        be used as solid catalyst. Accordingly, the coprecipitation        method has such a drawback that the production cost of a        catalyst becomes high.

Besides, in a method in which Ce ions are highly dispersed within microvoids of a zeolite, whereas a catalyst having an extremely high initialperformance can be obtained, the catalyst is easily deteriorated by theso-called de-aluminum phenomenon wherein the aluminum contained in azeolite is separated from the zeolite structure into the micro voids ofthe zeolite. Since the de-aluminum phenomenon is accelerated especiallyby a high ambient temperature and the presence of steam, when thecatalyst is used for a denitration at a high temperature wherein thecatalyst is exposed to an exhaust gas containing 5 to 10% of steam formany hours, it is difficult to maintain a high activity of the catalystfor a long period of time.

In consideration of the problems existing in the prior arts, the subjectmatters of the present invention is to provide a denitrating catalyst inwhich a Ce oxide is prevented from being embedded in a Ti oxide torealize such a high degree of dispersion of the Ce oxide on the surfaceof the Ti oxide as comparable with the case wherein cerium ions aredispersed within micro voids of a zeolite by ion exchange method, andwhich catalyst is free from occurring such phenomena as sintering of theTi oxide and deterioration of the zeolite with steam. For instance, thepresent invention is to provide a method by which NOx contained in ahigh temperature exhaust gas exhausted from such an equipment as thatfor electric power generation using simple cycle gas turbines is treatedto render into harmless.

DISCLOSURE OF THE INVENTION

As a result of repeated diligent researches conducted by the presentinventors to develop a catalyst containing a titanium oxide as a maincomponent and having a greatly increased heat resistance as well asactivity, the following conclusions were drawn:

-   -   i) In order to greatly increase the activity of the catalyst, it        becomes an efficient measure to highly disperse Ce ions or oxide        in micro voids, for example, micro voids of a zeolite, thereby        increase the contact of the Ce ions or oxide with an exhaust        gas.    -   ii) When a Ti compound and a Ce compound are mixed under a        condition wherein the compounds can easily be blended with a        high degree of dispersion, the blending proceeds too far so that        the Ce compound is embedded in the Ti compound (for example,        TiO₂) since both the compounds have some similarities in        properties between them. Thus, the contact of the Ce compound        with an exhaust gas is impeded and the Ce compound does not        function as active component. Not only that, sintering of TiO₂        is accelerated sometimes by the Ce compound. Therefore, the        mixture is not suitable for a catalyst to be used at a high        temperature.

Then, diligent researches were further continued by the presentinventors, and as a result, such facts as follows were found, leading tothe accomplishment of the present invention:

That is, when TiO₂ having hydroxyl groups on its surface is mixed withtungstic acid or a salt thereof in the presence of water, the hydroxylgroups of the TiO₂ are condensed with the tungstic acid or a saltthereof to form bridges between crystals of the TiO₂. When the crystalsare calcined, micro voids having a diameter of less than 50 Å comparablewith the micro voids in a zeolite are formed as shown in FIG. 1(a). In acase wherein a titanium oxide having such micro voids is formed, when asol of CeO₂ which has a size of fine particle but can not be penetratedinto the interior of the TiO₂ is coexisted, the CeO₂ enters as an activecomponent into the micro voids formed between crystals of TiO₂ separatedby a W compound as shown in FIG. 1(b), and thus a high heat resistanceand a high activity are imparted to a catalyst to be obtained.

In the catalyst of the present invention, crystals of TiO₂ are orientedand cohered while holding tungstic acid or tungsten trioxide (WO₃) inbetween the crystals to form micro voids and particles of Ce oxide(which is an active component) in the gaps (or spaces) between the TiO₂crystals as described in the diagram of a catalyst shown FIG. 1(b),thereby prevent the TiO₂ crystals from contacting with one another toimpede the growth of the TiO₂ crystals by heat, and achieve a highactivity of a catalyst comparable with a zeolite in which Ce ions aredispersed by ion exchange method, and further completely prevent suchzeolite from being deteriorated when exposed at a high temperature to agas containing steam.

In the present invention, it is necessary that particles of Ce compoundwhich is an active component exist in the gaps between crystals of TiO₂as shown in FIG. 1(b). On the contrary, when TiO₂ and Ce oxide are mixedunder a condition wherein the compounds are in a state of soluble saltor ion having a high reactivity, the Ce oxide becomes in a state whereinthe particles of Ce oxide are embedded in the TiO₂ so that the particlesof Ce oxide can not exist within the micro voids. Thus, only a catalysthaving low performances can be obtained.

The present invention is summarized as follows:

-   -   (1) A method for producing a catalyst used for removing nitrogen        oxides which method comprises dispersing a hydrated titanium        oxide or dried material thereof, tungstic acid or a salt        thereof, and cerium dioxide in a dispersion medium to form a        sol-like material, mixing the sol-like material with an aqueous        medium to form a catalyst slurry or paste, supporting the        catalyst slurry or paste on a catalyst carrier, and then        calcinating the catalyst carrier.    -   (2) The method for producing a catalyst used for removing        nitrogen oxides recited in paragraph (1) above wherein a        colloidal silica is further mixed to form the catalyst slurry or        paste.    -   (3) The method for producing a catalyst used for removing        nitrogen oxides recited in paragraph (1) or (2) above wherein        oxalic acid is still further mixed to form the catalyst slurry        or paste.    -   (4) The method for producing a catalyst used for removing        nitrogen oxides recited in any one of paragraphs (1) to (3)        above wherein inorganic short fibers are still further mixed to        form the catalyst slurry or paste.    -   (5) The method for producing a catalyst used for removing        nitrogen oxides recited in any one of paragraphs (1) to (4)        above wherein the catalyst carrier is a catalyst carrier made of        inorganic fibers, a catalyst carrier made of a ceramic, or a        catalyst carrier made of a metal.    -   (6) The method for producing a catalyst used for removing        nitrogen oxides recited in paragraph (5) above wherein-the        catalyst carrier made of inorganic fibers is a corrugated        honeycomb carrier prepared by subjecting a sheet of        silica-alumina type inorganic fibers to a corrugating        processing.    -   (7) The method for producing a catalyst used for removing        nitrogen oxides recited in paragraph (5) above wherein the        catalyst carrier made of a metal is a metal lath.    -   (8) A catalyst used for removing nitrogen oxides which catalyst        is produced by a method recited in any one of paragraphs (1)        to (7) above.    -   (9) A method for removing nitrogen oxides from an exhaust gas        containing the nitrogen oxides by using a catalyst recited in        paragraph (8) above in the presence of ammonia.    -   (10) The method for removing nitrogen oxides recited in        paragraph (9) above wherein the temperature of the exhaust gas        is 350 to 600° C.    -   (11) The method for removing nitrogen oxides recited in        paragraph (9) above wherein the exhaust gas is an exhaust gas        exhausted from a gas turbine.

Typically, the catalyst of the present invention can be prepared asfollows; i) an oxo-acid of tungsten (W) or its salt, and cerium dioxideare dispersed in a slurry of a hydrated titanium oxide such asmetatitanic acid and orthotitanic acid, its dried material, or a colloidsuch as a titania sol by an aqueous medium to form a sol, ii) a pHadjustor such as oxalic acid and acetic acid, and a binder such as asilica sol are added to the sol formed in i) above when necessary, iii)the sol is mixed with an aqueous medium to convert into a slurry orpaste (hereinafter referred to as catalyst slurry or catalyst paste),iv-a) a honeycomb-like catalyst carrier prepared by subjecting aninorganic fiber sheet to corrugating processing, a catalyst carriercomprising a non-woven fabric sheet made of inorganic fibers, ahoneycomb-like catalyst carrier made of a ceramic such as cordierite andalumina, a catalyst carrier comprising a net-like material such as awire netting and metal lath, or a catalyst carrier comprising a net-likematerial prepared by weaving inorganic fiber yarns such as E-glass fiberyarns into a net-like structure is impregnated with the catalyst slurryor catalyst paste, iv-b) the catalyst slurry or catalyst paste isapplied on the catalyst carrier, or iv-c) the catalyst slurry orcatalyst paste is applied heavily on the net-like carrier and pressed sothat the catalyst slurry or catalyst paste fills the meshes of the net,and then v) the catalyst carrier is dried and calcined.

A raw material for a titanium oxide used in the present invention may beany material so long as it has hydroxyl groups on the surface oftitanium oxide, and specifically, a hydrated titanium oxide, a sol-likematerial of a titanium oxide, and a dried material thereof can be used.As further example, the raw material may be one containing sulfuric acidradical as impurity, like a dried material prepared from metatitanicacid obtained by “sulfuric acid process”.

As a raw material for tungsten (W), an oxo-acid or heteropolyacidcontaining MO₄ type ion (M:W) of a corresponding metal (M), or anammonium salt such as ammonium meta- or para-tungstate can be used. Theamount of the raw material for tungsten to be added is 1 to 20 atom %and preferably 5 to 15 atom % of a catalyst. When the amount of tungstento be added is small, deterioration in the heat resistance of a catalystis caused, but when the amount is too large, the percentage of titaniumoxide used for maintaining the active component is decreased to producethe lowering of the activity of a catalyst.

On the other hand, a sol-like material of a cerium oxide is a dispersionof CeO₂ dispersed in water containing an organic alkali or acid asstabilizer, and a sol-like material currently on the market as a UVstabilizer or coating agent may also be used. Its amount to be added is0 to 10 atom % and preferably in the range of 1 to 5 atom % of acatalyst. When its amount to be added is too small, it is difficult toobtain a catalyst having a high activity, but when the amount is toolarge, deterioration in the activity of a catalyst at a temperaturehigher than 500° C. tends to be caused.

Whereas the addition of oxalic acid or acetic acid is not necessarilyrequired, they are added as occasion arises, when ammonim salt oftungstic acid is used, since the acid reacts with ammonium ion to formtungstic acid and accelerate the adsorption of tungstic acid to titaniumoxide. Particularly, since oxalic acid has a property of slightlydissolving titanium oxide and activating the surface of titanium oxideto accelerate the reaction with tungstic acid, it is easy to obtain apreferable result when 5 to 10% by weight of oxalic acid based on theweight of titanium oxide is added.

As a catalyst carrier for supporting a catalyst slurry or pastecontaining the compounds described above, a honeycomb-like carrierprepared by subjecting an inorganic fiber sheet to a corrugatingprocessing, non-woven fabric made of inorganic fibers, net-like materialsuch as a wire netting and metal lath, net-like product prepared byweaving inorganic fiber yarns such as E-glass fiber yarns into anet-like structure, and the like may be used as described above, and thecarriers may be used after reinforced by a known reinforcing agent, orafter provided with a coated layer effecting the purposes of increasingthe adhesibility of catalyst components and preventing a metal substratefrom being oxidized.

While the method for supporting a catalyst slurry or paste may be anyone, it is appropriate for a corrugated honeycomb made of inorganicfibers, ceramic non-woven fabric, and ceramic honeycomb carrier toimmerse the carrier in a slurry containing 30 to 50% by weight ofcatalyst components so that the catalyst slurry is filled in the gapsbetween fibers and coated on the surface of the carrier. On the otherhand, in the case wherein a metal or ceramic net-like product is used,when the size of its meshes is small, a method in which a paste preparedby adding inorganic fibers in a catalyst paste containing 30 to 35% byweight of water is applied on the net-like product by using a roller soas to fill the meshes can be adopted in addition to the coating methoddescribed above.

A catalyst precursor prepared by one of the methods described above andsupporting a catalyst slurry or catalyst paste on a various type ofsubstrate is dried by a known method such as air-drying and hot-airdrying, after subjected to treatments such as cutting, shaping, anddeformation when necessary, and then calcined at a temperature of 500 to700° C. and used as catalyst.

The catalyst of the present invention is preferably used for removingNOx from a high temperature exhaust gas, for example, an exhaust gashaving a temperature of 350 to 600° C., desirably 450 to 600° C., andmost preferably 500 to 600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for illustrating a catalyst structure by which theconcept of the present invention is made concrete.

FIG. 2 is a diagram for illustrating the structure of a conventionalcatalyst.

FIG. 3 is a graph in which the denitration ratios obtained by using anexample of the catalyst of the present invention and catalysts ofComparative Examples are compared to show an effect of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES

Now, the present invention will be described in more detail withreference to specific examples. However, it should be understood thatthe scope of the present invention is by no means limited by suchspecific-examples.

Example 1

Titanium oxide dried at a low temperature (produced by Millennium Co.,Ltd., trade name: G5, surface area: 275 m²/g) in an amount of 80 g, 52.7g of aqueous solution of ammonium metatungstate ((NH₄)₆.H₂W₁₂O₄₀.xH₂O,contained in the solution at a concentration of 50% by weight as WO₃),26.2 g of CeO₂ sol (produced by Taki Chemical Co., Ltd., trade name:Needrahl, CeO₂ content: 15% by weight, 4 g of oxalic acid, 50 g ofsilica sol (produced by Nissan Chemical Ind., trade name: OS sol,concentration: 20% by weight), and 50 g of water were mixed to prepare acatalyst slurry having a concentration of TiO₂ of 30% by weight.

In the catalyst slurry thus obtained was immersed a corrugated honeycombmade of aluminosilicate type inorganic fibers (produced by Nichias Co.,Ltd, trade name: No. 3722) and cut into 5 cm square so that the catalystslurry was supported between the inorganic fibers and on the surface ofthe fibers, dried at 150° C., and then calcined at 600° C. for 2 hoursto prepare a catalyst.

In this case, the amount of the catalyst supported was 300 g/L, and thecatalyst had a chemical composition of Ti/W/Ce (88/10/2) in terms ofatom percentage.

Examples 2 and 3

Catalysts were prepared by the same method as in Example 1 with theexception that the amounts of aqueous solution of ammonium metatungstateand CeO₂ sol added were changed respectively to 83.8 g and 27.6 g(Example 2), or 24.9 g and 24.8 g (Example 3) but the catalyst slurriesmaintained a TiO₂ concentration of about 30% by weight by adjusting theamount of water to be added in both Examples.

The catalysts thus obtained had chemical compositions of Ti/W/Ce(83/15/2 in Example 2 and 93/5/2 in Example 3) in terms of atompercentage, respectively.

Examples 4 through 6

Catalysts were prepared by the same method as in Example 1 with theexception that the amounts of aqueous solution of ammonium metatungstateand CeO₂ sol added were changed respectively to 52.1 g and 12.9 g(Example 4), 53.3 g and 39.7 g (Example 5 ), or 54.9 g and 67.8 g(Example 6 ) but the catalyst slurries maintained a TiO₂ concentrationof about 30% by weight by adjusting the amount of water to be added ineach Example.

The catalysts thus obtained had chemical compositions of Ti/W/Ce(89/10/1 in Example 4, 87/10/3 in Example 5, and 85/10/5 in Example 6 )in terms of atom percentage, respectively.

Comparative Example 1

A catalyst was prepared by the same method as in Example 1 with theexception that CeO₂ sol was not added.

Comparative Example 2

A catalyst having a chemical composition of Ti/W/Ce (88/10/2) in termsof atom percentage was prepared by the same method as in Example 1 withthe exception that the CeO₂ sol in Example 1 was replaced by 9.9 g ofcerium nitrate (Ce(NO₃ )₃.6H₂O).

Comparative Example 3

Hydrogen substitution type mordenite (produced by Tosoh Corp., Si/Alratio: 23.1) in an amount of 100 g and 9.3 g of cerium nitrate (Ce(NO₃)₃.6H₂O) were dispersed in 200 g of water, and the dispersion wassubjected to evaporation to dryness on a sand bath and then tocalcination at 550° C. for 2 hours to prepare a Ce substitution typemordenite (Ce ion 3% by weight). The powders of Ce substitution typemordenite thus obtained was dispersed in a mixed solution of 10 ng ofsilica sol (produced by Nissan Chemical Ind., trade name: OS sol,concentration: 20% by weight) and 10 ng of water to prepare a catalystslurry. In the catalyst slurry thus obtained was immersed a corrugatedhoneycomb made of aluminosilicate type inorganic fibers (produced byNichias Co., Ltd, trade name: No. 3722) and cut into 5 cm square so thatthe catalyst slurry was supported between the inorganic fibers and onthe surface of the fibers, dried at 150° C., and then calcined at 600°C. for 2 hours to prepare a catalyst.

With the catalysts prepared in Example 1, and Comparative Examples 1through 3, denitration ratios from an exhaust gas in the coexistence ofammonia under conditions shown in Table 1 at temperatures from 350 to.600° C. were determined, respectively. TABLE 1 Item Value 1. Spacevelocity SV 60,000 l/h 2. Gas composition NH₃ 240 ppm NO 200 ppm O₂ 10%CO₂  5% H₂O  5%

The results thus obtained are shown in FIG. 3. From the results shown inFIG. 3, it can be understood that the catalyst prepared in Example 1 ofthe present invention has an extremely high performance compared withthe catalyst having a chemical composition of Ti/W and prepared inComparative Example 1 and the catalyst prepared in Comparative Example 2wherein cerium nitrate was used as raw material for Ce, and has a highactivity comparable with the performance of the Ce substitution typezeolite prepared in Comparative Example 3.

On the other hand, with the catalysts prepared in Examples 1 through 6and Comparative Examples 1 through 3, heat resistance tests whereinsample catalysts are maintained in the atmosphere at 550° C. for 200hours, and steam resistance tests wherein sample catalysts aremaintained in a gas containing 30% of H₂O and shown in Table 2 at 550°C. for 200 hours were carried out. With the catalysts after these tests,denitration ratios were determined under the conditions shown inTable 1. Among the performances (denitration ratios) obtained, thevalues determined at the initial stage, after the heat resistance tests,and after the steam resistance tests at 550° C., respectively, are shownin Table 3 for comparison. TABLE 2 Item Value 1. Space velocity SV60,000 l/h 2. Gas composition O₂ 20% H₂O 30%

TABLE 3 Initial After heat After steam Catalyst stage resistance testresistance test Example 1 85.6 83.2 84.5 Example 2 79 79.2 79.7 Example3 82.1 82.7 82.9 Example 4 83.1 81.4 82 Example 5 80.5 79.8 78.8 Example6 78 78.1 77.9 Comparative 65.6 63.2 63.5 Example 1 Comparative 70.171.3 69.4 Example 2 Comparative 93.8 91.5 32.2 Example 3

As will be clear from Table 2 and Table 3, the catalysts prepared inExamples 1 through 6 of the present invention are not only high in theirperformances compared with those of the catalysts prepared inComparative Examples 1 and 2, but also have excellent durability ofcausing almost no deterioration in the activity by subjecting thecatalysts to the heat resistance tests and steam resistant tests. Incontrast, whereas the Ce substitution type mordenite catalyst preparedin Comparative Example 3 had values as excellent as the catalysts of thepresent invention in the initial performance and after heat resistancetest, the catalyst of Comparative Example 3 remarkably lowered in theactivity not only to show an extremely low value compared with thecatalysts of the Examples of the present invention but also to show thelowest value in the catalysts of Comparative Examples after the steamresistance test. From these results, it can be said that the catalystsof the present invention not only have a high denitration performance athigh temperatures but also are excellent in heat resistance and steamresistance, demonstrating the effectiveness of the concept of thepresent invention.

On the other hand, when the results obtained in Examples 1, 2, and 3 arecompared, it can be understood that the performance (denitration ratio)of a catalyst tends to somewhat decrease when the content of W in acatalyst is small or too large. Further, from the comparison of theresults obtained in Example 1, and 4 through 6, it can be seen that whenthe amount of Ce added is too large, heat resistance of a catalyst tendsto decrease and the amount of Ce to be added is preferably less than 5atom percentage.

Example 7

Titanium oxide (produced by Ishihara Sangyo, specific surface area: 250m²/g) in an amount of 15 kg, 9.7 kg of ammonium metatungstate, 4 kg ofCeO₂ sol, 0.8 kg of oxalic acid, and 2 kg of water were charged into akneader, kneaded for 20 minutes, and further kneaded for 30 minuteswhile gradually adding 4 kg of silica-alumina type ceramic fibers(produced by Toshiba Corp., trade name: Fineflex) thereto to obtain acatalyst paste having a water content of 32%. The catalyst paste thusobtained was placed on the surface-of a substrate prepared by subjectinga SUS304 steel plate having a thickness of 0.2 mm to metal lathprocessing, held between two polyethylene sheets, and then passedthrough a pair of press rollers so that the catalyst paste was filled inmeshes and applied on the surface of the metal lath substrate. Thesubstrate was air dried and then calcined at 600° C. for 2 hours toobtain a plate-like catalyst.

When the catalyst thus obtained was subjected to determination ofdenitration ratio under a condition of aerial velocity of 51 m/h at 550°C., the nitration ratio was 75%. This value corresponds to 83% ofdenitration ratio at a SV of 60,0001/h obtained in Example 1 and showsthat the catalysts prepared according to the method of the presentinvention have high performances at high temperatures despite the methodfor supporting a catalyst on a carrier.

Industrial Applicability

According to the invention defined in any one of claims 1 through 7, acatalyst can be obtained which is used for removing nitrogen oxides, hasa high activity and heat resistance, and makes efficient removal of NOxcontained in a high temperature exhaust gas such as an exhaust gas froma gas turbine having no HRSG (heat recovery steam generator) forpurifying the exhaust gas possible. Besides, in the method of thepresent invention for producing a catalyst, such a complicated step ascoprecipitation is not required in precipitating operation, and thecatalyst can be obtained through extremely small number of steps ofsupporting a catalyst slurry or paste on a variety of carrier afterblending catalyst raw materials. This fact leads to a social effect ofproducing a low-priced but excellent apparatus for purifying an exhaustgas to contribute to the improvement of environment.

According to the invention defined in claim 8, a compact denitratingapparatus can be realized by using a catalyst having a high activity,and it is possible to decrease the size of a reactor made of expensivehighly heat resistant material and to save the weight of the reactor.

According to the invention defined in any one of claims 9 through 11, itis possible to efficiently remove nitrogen oxides from a hightemperature exhaust gas.

1. A method for producing a catalyst for removing nitrogen oxides whichcomprises dispersing a hydrated titanium oxide or dried materialthereof, tungstic acid or a salt thereof, and cerium dioxide in adispersion medium to form a sol-like material, mixing the sol-likematerial with an aqueous medium to form a catalyst slurry or paste,supporting the catalyst slurry or paste on a catalyst carrier, and thencalcinating the carrier.
 2. The method for producing a catalyst forremoving nitrogen oxides according to claim 1 wherein a colloidal silicais further mixed to form the catalyst slurry or paste.
 3. The method forproducing a catalyst for removing nitrogen oxides according to claim 1wherein oxalic acid is still further mixed to form the catalyst slurryor paste.
 4. The method for producing a catalyst for removing nitrogenoxides according to claim 1 wherein inorganic short fibers are stillfurther mixed to form the catalyst slurry or paste.
 5. The method forproducing a catalyst for removing nitrogen oxides according to claim 1wherein the catalyst carrier is an inorganic fiber catalyst carrier,ceramic catalyst carrier, or metal catalyst carrier.
 6. The method forproducing a catalyst for removing nitrogen oxides according to claim 5wherein the inorganic fiber catalyst carrier is a corrugated honeycombcarrier prepared by subjecting a sheet of silica-alumina type inorganicfibers to a corrugating processing.
 7. The method for producing acatalyst for removing nitrogen oxides according to claim 5 wherein themetal catalyst carrier is a metal lath.
 8. A catalyst for removingnitrogen oxides which catalyst is produced by a method defined inclaim
 1. 9. A method of removing nitrogen oxides from an exhaust gascontaining the nitrogen oxides by using a catalyst defined in claim 8 inthe presence of ammonia.
 10. The method for removing nitrogen oxidesaccording to claim 9 wherein the temperature of the exhaust gas is 350to 600° C.
 11. The method for removing nitrogen oxides according toclaim 9 wherein the exhaust gas is an exhaust gas from a gas turbine.12. A catalyst for removing nitrogen oxides which catalyst is producedby a method defined in claim
 2. 13. A catalyst for removing nitrogenoxides which catalyst is produced by a method defined in claim
 3. 14. Acatalyst for removing nitrogen oxides which catalyst is produced by amethod defined in claim
 4. 15. A catalyst for removing nitrogen oxideswhich catalyst is produced by a method defined in claim 5.